Bully is a rat who appears as an enemy of the Farthing Wood Animals in both the books and the TV series. He is the leader of the brown rats who attempt to take over White Deer Park in the books and the TV series. In the books, Bully emerges as the leader quite late on, when the rats are struggling to take over White Deer Park due to the attacks by the park's residents. In the TV series, he is shown to already lead the rats as soon as they arrive at the reserve. Bully is the main antagonist of series 3.

Bully is an extremely vicious and threatening character in the books, but in the TV series he is considerably less threatening and more comedic, as well as more caring, affectionate, and sympathetic of his fellow rats (but still very vicious). In the TV series, he has an unusually long tail (even for a rat) which he often waves to alert the other rats and uses it as a whip to attack them and other creatures. He also constantly uses the phrase "Who am I?", to which the other rats must reply "Bully, Bully, Bully". In the books, he is killed by Vixen during the final battle to take over White Deer Park, but in the TV series he has his tail bitten off by Cleo and retreats with the other rats back to the sewers.

Bully emerges as the leader of the rats while they are fighting their battle against the Farthing Wood animals. He tells the other rats that if they keep multiplying and work together they will soon be able to take over the whole reserve, and not even the Farthing Wood animals will be able to stop them. When the rats invade the pond and Toad tries to stand up to them, Bully tells them that the rats cannot be stopped, before fatally injuring Toad by biting him all over his body.

Bully and several other rats later ambush Adder as the snake attempts to attack one their nests. They nearly kill him, but Plucky arrives and forces the rats to release Adder by threatening to dig into the nest. Bully then proposes to Plucky that there should be a temporary truce between the rats and the Farthing Wood animals and although Plucky does not immediately agree, Bully is satisfied that they will not attack for the time being. However, the wily rat secretly plans to launch a full force attack on the residents of the reserve while their guard is down. Bully puts together a hunting party of the largest rats to dispose of Adder; they instead find Sinuous, Adder's mate, mistaking her for Adder, and attack and kill her.

Later, Bully gathers all the rats together, forming a huge army. He then launches an attack on the Farthing Wood animals, which the rats appear to be winning early on until the other residents of the reserve arrive to help fight them off. When this happens, Bully tries to escape, but Vixen pursues him and crushes him in her jaws before throwing his body over the boundary fence, thus depriving the rats of their leader and avenging the death of Toad.